Charged particle beam columns are typically employed in scanning electron microscopy (SEM), which is a known technique widely used in the manufacture of semiconductor devices, being utilized in a CD metrology tool, the so-called CD-SEM (critical dimension scanning electron microscope) and defect review SEM. In a SEM, the region of a sample to be examined is two-dimensionally scanned by means of a focused primary beam of electrically charged particles, usually electrons. Irradiation of the sample with the primary electron beam releases secondary (and/or backscattered) electrons. The secondary electrons are released at that side of the sample at which the primary electron beam is incident, and move back to be captured by a detector, which generates an output electric signal proportional to the so-detected electric current. The energy and/or the energy distribution of the secondary electrons is indicative of the nature and composition of the sample.
SEM typically includes such main constructional parts as an electron beam source (formed with a small tip called “electron gun”), an electron beam column, and a detector unit. The electron beam column includes inter alia a gun alignment system, a beam blank means, a beam axis alignment system (usually called “aperture alignment coils”), a beam shaper system (typically a “stigmator” composed of one or more quadruple lenses), and a focusing assembly (including an objective lens arrangement) and one or more deflectors. A primary electron beam propagating towards a focusing assembly undergoes beam axis alignment while passing through the alignment coils, and is then affected by a stigmator.
The alignment of the primary beam axis is typically aimed at correcting various aberrations of focusing, and consists of deflecting the primary beam axis to ensure that the beam axis passes through a specific point of the objective lens arrangement, usually called the “central” point thereof. This “specific point” is such that changing the energy of a beam that passes through this point in the objective lens arrangement will not cause the beam deflection by the objective lens arrangement. Generally speaking, the beam axis should be aligned with respect to the optical axis of the objective lens arrangement so as to ensure minimal spot-size imaging of the cathode-tip onto the sample's surface. A stigmator, in turn, typically creates a magnetic or electrostatic field affecting the cross section of the primary beam to compensate for aberrations of focusing caused by astigmatism effect of the objective lens arrangement (e.g., axial asymmetry of the focusing field of the objective lens arrangement), and to ensure a substantially circular cross section of the primary beam that has been focused onto a sample by the focusing assembly.
The detector unit may be located outside the path of the primary beam propagation through the column. In this case, a generator of orthogonal electric and magnetic fields (known as Wien-filter) is employed to direct secondary electrons to the detector (e.g., U.S. Pat. Nos. 5,894,124; 5,900,629). To ensure detection of those secondary electrons that are not sufficiently deflected by the Wien-filter, a target or extracting electrode made of a material capable of generating a secondary electron when an electron collides therewith is additionally used. Such a target is formed with an aperture and is located such that the axis of the primary beam propagation towards the focusing assembly intersects with this aperture, which thereby serves as a primary beam hole.
As disclosed in WO 01/45136 assigned to the assignee of the present application, the detector unit includes a detector having a primary beam hole that is located in the path of a primary electron beam propagating towards the focusing assembly. Here, a deflection system associated with the focusing assembly operates to affect the trajectory of the primary electron beam such that the primary electron beam impinges onto a sample along an axis forming a certain angle with the sample's surface (the so-called “tilt mode”). The tilt mode is usually utilized to inspect samples that have a surface relief, i.e., pattern in the form of a plurality of spaced-apart grooves to detect the existence of a foreign particle located inside a narrow groove.
Generally, a tilt mechanism can be implemented by mechanically tilting either the sample carrier relative to the charged particle beam column (e.g., U.S. Pat. Nos. 5,734,164; 5,894,124; 6,037,589) or the column relative to the sample's stage (e.g., U.S. Pat. No. 5,329,125). According to the above-indicated technique of WO 01/45136, a tilt mechanism is achieved by affecting the trajectory of the primary electron beam using single- or double-deflection. The use of a double-deflection technique, namely, pre-lens and in-lens deflection stages, for stereoscopic visualization of a sample is also known, being disclosed, for example, in EP 1045426.